150 WPF means that your accuracy with Hunting Crossbow and Light Crossbow is at its maximum. The Crossbow's accuracy is nearly at its maximum, and Heavy Crossbow and Arbalest will gain much more precision if you invest more into your WPF. What feels fine to you depends obviously on the crossbow you pick, and on your play style. If you are skirmishing, you might not need more. If you are going to stay far away from the enemies you shoot, it is better to increase your WPF and go for Heavy Crossbow or Arbalest.
About the shield, all I could do is list you some choices. But the decision is up to you. Good luck to do the right thing, and whatever you pick, try not to regret it later.
If you use 149 WPF in twohanded, use the formula
raw damage before armor = 1.5029*(weapon damage)+3
If you use 180 WPF, use the formula
raw damage before armor = 1.568*(weapon damage)+3
The raw damage before armor is the damage a naked enemy would suffer when your swing has no speed bonus or malus. Since your PS is fixed at 5 and the WPF is either 149 or 180, I simplifed the general raw damage before armor formula to
weapon damage as the only input variable. Keep in mind though that wearing heavy armor will decrease your raw damage before armor slightly.
The speed increase from 149 to 180 WPF is approximately 2.015%. Keep in mind that heavy armor will make your swings a bit slower.
Can I have a cookie now?